Correlation techniques used in conjunction with noise codes have been utilized in the past in signal processing systems. These noise coded systems include, for example, over the horizon systems employing various types of scatter techniques, satellite communications systems, and the like, and multiple access systems employing address codes to enable utilization of the system. One such system is shown and described in U.S. Pat. No. 3,908,088, entitled, "Time Division Multiple Access Communications System", issued to Frank S. Gutleber, the present inventor, on Sept. 23, 1975.
The correlation and coding techniques employed in this type of communications system results in increased signal-to-noise ratios without any increase of transmitter power. It not only operates to minimize multipath effects, but also obviates interference or cross-talk between the channels while operating with overlapping noise coded signals. Typically, the technique employed utilizes a passive matched filter which pulse compresses a wide pulse to a narrow pulse whose peak amplitude is increased by the number of code bits present in the processed code. Accordingly, the output comprises a single peak of relatively high amplitude having a pulsewidth substantially narrower than the pulsewidth of the received signal without spurious peaks of lower amplitude elsewhere in the waveform.
Furthermore, one class of noise codes are known wherein pairs of coded signals termed "code mates" have autocorrelation functions which upon detection with a matched filter provide a peak output at a given time and a zero output or outputs having the same amplitude with the opposite polarity at all other times. When the code mates are detected and the resultant detected outputs are linearly added, there is provided an impulse output of high amplitude at a given time and a zero output at all other times. Typical examples of means for generating such codes and the utilization thereof in communications systems is typically shown in the following patents issued to the present inventor: U.S. Pat. No. 3,461,451, entitled, "Code Generator To Produce Permutations Of Code Mates", Aug. 12, 1969; U.S. Pat. No. 3,519,746, entitled, "Means And Method To Obtain An Impulse Autocorrelation Function", July 7, 1970; and U.S. Pat. No. 3,634,765, entitled, "System To Provide An Impulse Autocorrelation Function . . . ", Jan. 11, 1972.
Accordingly, it is an object of the present invention to provide an improvement in pulse code communications systems.
Another object of the present invention is to provide an improvement in multichannel time division multiplexed trunk transmission links.
Still another object of the present invention is to provide an improvement in a multichannel time division multiplexed trunk transmission link which provides a large degree of interference or jamming rejection by employing code mates for selected channels which when detected in a matched filter compress to a single impulse containing substantially no side lobes for each selected channel.